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Old 11th April 2005, 06:40 AM   #1
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Join Date: Mar 2005
Location: Canada
Default Trouble with speaker protection circuit (Randy Slone's circuit)

It's the figure 8.5 protection circuit the 'Audiophile's project sourcebook'.
The problem is that the LED is always flashing rapidly indicating a thermal overload. This is without a relay or an amplifier output connected.
Since I'm using a transformer with 40-0-40 output, I changed R1 to 720ohms. Also Q4(2SD669) to the MJE340 since I figure it's only being used as a switch.

I have made some voltage measurements.
Voltage across C4 is about 6V, which should be enough to saturate the darlington pair.
Voltage across the relay outputs 0V.
Supply voltage of the multivibrator circuit 26V.
V-D2 (Cathode side to common) 22V.

I have checked the resistor values as well as doing the diode check on the transistors.

So far with my limited electronic experience this is about all I can do, any help would be greatly appreaciated .

I noticed alot of the guys here are from Vancouver, BC. I just found out my co-worker also visits this site.
Maybe we can get together for a little amplifier meet.
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Old 12th April 2005, 12:39 AM   #2
moamps is offline moamps  Croatia
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Join Date: Oct 2002
Location: Croatia
Default Re: Trouble with speaker protection circuit (Randy Slone's circuit)

Quote:
Originally posted by whalefat
.....Voltage across C4 is about 6V, which should be enough to saturate the darlington pair.....
Hi,
Voltage across C4 should be about 1.4V. Check relay driver transistors with multimeter and also check transistors wiring. Btw, MJE 340 has different pinning than 2SD669.

Regards,
Milan
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Old 12th April 2005, 06:49 AM   #3
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Join Date: Mar 2005
Location: Canada
I fixed it, thanks for getting my attention to the MJE340 transistor.
It turns out I just soldered it backwards.
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Old 13th April 2005, 10:13 AM   #4
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Join Date: Jun 2004
Location: HKSAR
Hi whalefat,

Can you please post the schematic? I went to Sloan's site (Seal Electronics) but now its 'consumed'.
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